(1) Field of the Invention
The present invention relates to a method of recovering acetic acid from an acetic acid-containing aqueous solution. More particularly, the present invention relates to a method of recovery of acetic acid which comprises in combination the extraction step using an extracting agent including a tertiary amine and the dehydration and recover step utilizing distillation.
(2) Description of the Prior Art
Acetic acid, acetic anhydride and peracetic acid are used in the organic chemical industries, for example, for manufacture of cellulose acetate, alkyl acetates, ketenes, glycerin and epoxyalkanoic acids, and acetic acid-containing solutions are withdrawn from these processes. For example, acetic acid is formed as a by-product at a concentration of 20 to 40% from the process for production of cellulose acetate, and acetic acid is formed as a by-product at a concentration of 10 to 15% from the process for production of glycerin by the use of peracetic acid. Aqueous solutions containing acetic acid at such medium concentrations of about 7 to about 40% are produced in large quantities as by-products, and effective recovery of acetic acid from such aqueous solutions are indispensable and important for increasing economical efficiencies of the main processes. Furthermore, acetic acid is used in other fields, for example, in the metal treatments industries and in the fermentation industries, and also in these fields, there are formed aqueous solutions containing acetic acid. In order to enhance the utilization ratios of valuable substances and prevent occurrence of environmental pollution, it is very important to recover acetic acid at high efficiencies from these acetic acid-containing solutions.
An extraction method using an organic solvent is known as means for recovering a carboxylic acid from an aqueous solution, and this method can be applied to the recovery of acetic acid. Among carboxylic acids, acetic acid has a high affinity with water, and although various compounds have been used as extracting agents for the recovery of acetic acid, none of them have proved to provide satisfactory results. More specifically, the distribution coefficient, which has significant influences on the efficiency of the recovery by extraction, is ordinarily very small in case of acetic acid, and therefore, in order to increase the extraction ratio, it is necessary to use a large amount of a solvent, which results in increase of the energy consumption at the separation step.
Distribution coefficients to dilute aqueous solutions of acetic acid, shown by Treybal and Won, which are cited by J. M. Wardell and C. J. King in J. Chem. Eng. Data, 23 (2), 144 [1978], are as follows:
______________________________________ Ethers (C.sub.4 -C.sub.8) 0.63-0.14 Acetates (C.sub.4 -C.sub.10) 0.89-0.17 Ketones (C.sub.4 -C.sub.10) 1.20-0.61 Alcohols (C.sub.4 -C.sub.8) 1.68-0.64 ______________________________________
Among these extracting agents, ethyl acetate has a relatively large distribution coefficient to acetic acid (0.9 to 1.0) and is easily available, and therefore, ethyl acetate is customarily used as the extracting agent for recovery of acetic acid. However, since the boiling point of ethyl acetate is lower than that of acetic acid, all of the solvent used in a large amount should completely be evaporated. Furthermore, a large amount of water is dissolved in the extract and loss by dissolution in water is considerable. Accordingly, ethyl acetate as the extracting agent is unsatisfactory also with respect to the mutual solubility with water.
There has been proposed an extraction method using an extracting agent having a boiling point higher than that of acetic acid. If there is a solvent having a distribution coefficient comparable to that of a low-boiling-point solvent, which is applicable to this extraction method, since all of the solvent used need not be evaporated, this method is economically advantageous because the energy consumption is reduced. However, it the boiling point of the solvent is too high, the column bottom heating temperature cannot sufficiently be provided by steam ordinarily available in a chemical plant. If reduced pressure distillation is carried out for overcoming this disadvantage, condensation of the distillate cannot sufficiently be accomplished by ordinary cooling water.
These difficulties involved in heating and cooling can be eliminated by using a solvent having a boiling point higher than that of acetic acid but lower than 150.degree. C., such as isoamyl acetate, as disclosed in U.S. Pat. No. 1,839,894. However, separation of the solvent from acetic acid becomes difficult, and the amount of the solvents dissolved in water is increased.
As will be apparent from the foregoing description, satisfactory recovery of acetic acid cannot be attained by any methods based on the principle of the physical distribution of acetic acid between the organic phase and the aqueous phase.
There also is known a method in which an acid in an aqueous solution is extracted with an amine, which is a basic organic liquid, by the use of chemical reaction. For example, in the field of a chemical treatment of a nuclear fuel or a wet refining treatment of a metal, a high-molecular-weight amine, together with a diluent such as kerosene or an aromatic hydrocarbon, is used for extraction of an inorganic acid.
Such waste acid treatment using an amine has been known in the field of metal industries. However, separation of an acid from an extract is ordinarily accomplished by stripping with water, and it is not true that water-free phosphoric acid or sulfuric acid is recovered by the above method. In this point, recovery of inorganic acids is different from recovery of acetic acid in organic chemical industries where acetic acid should be recovered in the for of a water-free pure product.
Inoue et al. have published the results of researches made on the equilibrium of extraction of acetic acid with a high-molecular-weight amine and the extraction speed [Kagaku Kogaku, 33, page 1221 and Kagaku Kogaku Ronbun Shu 5 (2), page 212]. The amine used by Inoue et al. is a high-molecular-weight secondary amine having 24 to 27 carbon atoms as a whole, namely N-lauryl-(trialkylmethyl) amine (LA-2), and this amine is used in combination with a solvent such as chloroform, carbon tetrachloride, MIBK, n-hexane or cyclohexane.